Process for electrolytic manufacturing of alkali metal chlorates



April 1965 A. A. SCHWANBOM ETAL 3,

PROCESS FOR ELECTROLYTIC MANUFACTURING OF ALKALI METAL CHLORATES Filed Oct. 17, 1961 INVENTORS ANDECS 141/2910 1615 40/80 BY BIA/G7 Z/Df/v United States Patent Sweden Filed Oct. 17, 1961, Ser. No. 145,679 Claims priority, application Sweden, Oct. 18, 1960, 9,951/ 60 4 Claims. (Cl. 204-95) In manufacturing of alkali metal chlorates by electrolysis of an aqueous solution of alkali metal chloride considerable quantities of hydrogen gas are formed as byproduct. By various side-reactions oxygen and chlorine are formed moreover and if the electrodes are of graphite also carbon dioxide and traces of carbon monoxide develop which will pollute the hydrogen gas. Thus about 600 standard cub. meters of hydrogen are produced per ton of sodium chlorate and even under favourable technical operation conditions this hydrogen is polluted by 4-6% of oxygen, about 1% of carbon dioxide and 0.20.6% of chlorine.

The gas mixture thus obtained is of a composition just above the upper explosive limit which is about 94% of hydrogen for a mixture of pure hydrogen and oxygen, and in the present case has about the same rate. To avoid explosions safely it is conventional to introduce a volume of air into the gas space of the electrolytic cell in order to decrease the hydrogen content below the lower explosion limit i.e. about 4%. As the explosion range then is passed a thorough and rapid admixture and a sufiicient volume of air is required to avoid formation of explosive gaseous mixtures. After washing the hydrogen gas thus diluted is allowed to escape to the atmosphere.

It is also possible to carry out the electrolysis without introducing air. Then it is necessary to check the tightness of the electrolytic cells and the connecting pipelines and the evolved gas is allowed to create a certain overpressure within the cells. Accordingly it can be avoided that air gets in and the gas mixture composition is kept slightly above the upper explosive limit. This method, however, is hazardous because the content of dangerous impurities in the hydrogen gas can be increased owing to irregularities in the electrolytic process. An accidental interruption of the electric current supply can cause a pressure reduction and back-flow in some parts of the equipment or the air can get in at the bushings through the cell walls. An electrolytic cell of conventional construction of a manufacturing capacity of one ton chlorate per 24 hours may be provided with 2,500 bushings and consequently it is diflicult to control and maintain the tightness of each of these bushings. For the same reason it is also impossible to keep such an overpressure within the cells that the encased gas volume can serve as a safeguard at shorter shutdowns.

From the gas explosions reported by practical experiences it is apparent that the chances of risks mentioned are not to be neglected. As far as we know it therefore has not been possible to utilize the gas mixture. After removal of chlorine by washing the mixture is permitted to escape to the atmosphere.

The present invention relates to a process to carry out the chlorate electrolysis safely without introducing air into the electrolytic cells. By this process moreover the "ice evolved hydrogen can be utilized in an economic and safe way for other valuable purposes e.g. for hydrogenation and for synthesis of ammonia or methanol.

The process according to the invention is characterized thereby that the electrolysis is carried out in closed cells provided with bipolar electrodes and a diluting gas is added under such conditions that the gas pressure within the cells always is higher than the atmospheric pressure. By this the air passage into the cells is avoided which is particularly significant e.g. if the electricity supply is interrupted. The diluting gas used preferably is hydrogen but other gases as well can be used which do not react with the gas formed at the electrolysis or with the electrolyte liquid; for example nitrogen and carbon dioxide. According to an embodiment of the invention the gas produced is reentered to the electrolytic cells after purification as described below.

It is not necessary that the gas added is absolutely pure but it should answer the above requirements. If hydrogen is used it may contain oxygen at the most in such an amount that the H content of the gas is substantially higher than the upper explosion limit. The amount of gas to be added has to be adjusted so that the hydrogen content of the gaseous mixture formed exceeds the upper explosion limit. To achieve sufficient security such a quan-' tity of gas has to be added that the content of 0 plus C1 in the mixture never exceeds 6%, preferably not more than 3%. The content tolerated in practical operation depends on the operating conditions (for example the temperature and moisture content of the gas) and besides on the sensitiveness and rapidity of the measuring and control devices used for the regulating of the gas supply. A suitable overpressure to be used in practice has turned out to be at least 10 mm. water pressure and the overpressure may be as high as 300 mm. W.P. The overpressure to be chosen depends primarily on the tightness of the electrolytic cells; when the overpressure is too high chlorine gas can leak out to the workshop which makes the stay there for the workers unpleasant and even unsanitary. On the other hand the overpressure should be as high as to permit the measuring and control devices to give the necessary impulses for compensation of rapidly occurring disturbances. For this reason the overpressure within the electrolytic cells should be at least 40 mm. W.P. It is obvious that such an overpressure is not operable in conventional chlorate electrolytic cells because of gas leakage. Using electrolytic cells with bipolar electrodes we have found that a sufiicient overpressure can be kept in practice, the number of bushings through the cell walls for the electric current supply being only part of What is required for conventional electrolytic cells and the gas leakage has been found quite negligible. The gas pressure within the electrolytic cell can be controlled by means of a valve in the exit pipeline and the valve can be controlled by a manometer in the cell.

The diluting gas is preferably introduced into the gaseous space above the electrolyte level. It also is possible to bubble in the gas through the electrolyte but the electric current consumption per ton chlorate produced is then higher. A simple embodiment is to introduce the gas into the pipeline system connected with the electrolytic cell through which the by-product hydrogen is withdrawn but this method will give incomplete security against explosions.

The mixture of introduced and evolved gas leaving the electrolytic cells is of such a composition that cannot be used directly for any technical purpose. It therefore is necessary to purify the mixture from one or several of the impurities, primarily from oxygen, chlorine and/or carbon dioxide. This can be done in any conventional manner. Chlorine and carbon dioxide are removed for example by treating the gaseous mixture with an alkaline solution which then may be added to the electrolyte. The oxygen is removed by passing the gaseous mixture over a combustion catalyst. The purification can be carried out more or less completely and a final gas of a purity for any specific purpose can be obtained. If the gas introduced into the electrolytic cells is pure hydrogen a final gas comprising hydrogen of same purity is obtained (but obviously a considerably greater volume). If the diluting gas is nitrogen a hydrogen-nitrogen mixture is obtained which can be used for instance for synthesis of ammonia.

Example 1 An electrolysis of a sodium chloride solution was carried out in a closed electrolytic cell provided with bipolar electrodes connected in series. Without introducing any diluting gas the oxygen content of the exit gas was about 4%, however at one occasion it increased to slightly over 6%. As it could not be excluded that the oxygen content occasionally and beyond control could reach an even higher value it was considered too dangerous to continue the electrolysis without applying the special safety measures according to this invention. Therefor hydrogen was introduced into the cell in a quantity corresponding to 850 m. per ton simultaneously formed chlorate. Operating thus for a considerable time proved that the content of plus C1 in the exit gas never exceeded 2.9%. The quantity of hydrogen added in a following operation period was reduced to 300 m. per ton chlorate. The exit gas then was of the following average composition:

At one occasion the maximum content of 0 plus C1 was 4.6%.

During the electrolysis the gas pressure in the cell was varied from the atmospheric pressure to an overpressure of some hundred millimeters W.P. without any significant advantage or disadvantage at any of the pressures chosen. The risk for air getting in the cell at shutdowns is of course lesser when a higher pressure is used but on the other hand the requirements as to the construction of the electrolytic cell and the operating control are higher in order to avoid that the gas leakage should become uneconomic or even unsanitary. The gas overpressure chosen for the following commercial operation was 110 mm. W.P. or slightly lower whereby the gas leakage from the economical point of view was inconsiderable and from the sanitary point of view it was harmless.

Example 2 An electrolysis was carried out in a cell of the same construction as described in Example 1 combined with purification of the exit gases from the cell. The principles of the equipment are illustrated in the accompanying drawing.

The electrolytic cell 1 is provided with gauges 2 and 3 for indicating the 0 content (possibly also the Cl -content) and the gas pressure. These gauges give impulses to the regulating devices controlling the quantity of gas to be introduced into the electrolytic cell and the pressure in the cell. The exit gas is led to a gas holder 4 wherefrom it is led for instance by means of a fan 5 through a washtower 6 sprayed by an alkaline solution. The washing liquid is circulated until its content of chloride plus hypochlorite (plus carbonate) has increased above a certain level, whereupon the solution is withdrawn from the washing system and replaced by fresh solution. The spent solution can be entered into the cell for utilizing of its content of combined chlorine.

To prevent the fan from emptying the gas holder 4, if for any reason the gas supply from the electrolytic cell should lapse, the fan 5 is provided with a by-pass conduit 7 wherein the valve automatically is opened if the gas volume in the gas holder falls below a certain level. The gas is led from the washtower through a container 8 provided with active carbon to absorb any impurities which could poison the catalyst in the following desoxidation process. The gas thus purified is led via the back pressure valve to the catalyst chamber 9 provided with a heat exchanger. The oxygen and hydrogen of the gas react over the platinum catalyst, the exit gas being practically pure hydrogen and moisture.

The hydrogen gas is cooled by means of water in a tubular or spray cooler 10 and then passed to the gas holder 11. The hydrogen can without further purification be utilized eg for hydrogenation of organic compounds, such as for preparation of alcohols from aldehydes or esters of fatty acids.

A not surplus of about 550 m. pure hydrogen was received per ton chlorate produced when the gas supply was automatically adjusted so that the content of 0 plus C1 in the exit gas from the electrolytic cell was 3%.

Example 3 An electrolysis was carried out according to Example 2 with the modification that the hydrogen gas purified was introduced into the electrolytic cell. The gas holder 11 was provided with a pipe to the electrolytic cell 1. The hydrogen supply source previously used was still connected to the cell as an emergency supply and was used occasionally when an adjustment of the system was carried out as often is required in experimental operation. Operating the cells and the hydrogen circulating system about 550 m. pure hydrogen was obtained per ton produced sodium chlorate which hydrogen could be withdrawn from the gas holder 11 and utilized for other purposes.

Example 4 An electrolysis was repeated as described in Example 2 whereby in. nitrogen gas per ton chlorate was introduced into the cell instead of hydrogen. The content of 0 plus C1 in the exit gas was 3.5-max. 5.0%. The composition of the purified gas mixture was within small tolerances 75% of hydrogen and 25% of nitrogen which mixture could be used for the synthesis of ammonia.

Example 5 An electrolysis according to Example 4 was repeated with the modification that 190 m. of nitrogen and about 190 m. of the purified exit gas were introduced into the cell. The average content of 0 plus C1 in the raw exit gas was less than 3% and the composition of the purified exit gas was the same as in Example 4, viz. 75% of hydrogen and 25% of nitrogen.

What we claim is:

1. A process for manufacturing alkali metal chlorates by electrolysis of an aqueous solution of an alkali metal chloride which comprises conducting the electrolysis in a closed electrolytic cell, introducing and passing through the cell a diluting gas at such a rate that oxygen and chlorine produced during electrolysis are maintained in a concentration of below 6 percent by volume while maintaining a gas pressure in the cell of at least 40 millimeters water pressure higher than atmospheric pressure, said diluting gas being at least in part gaseous hydrogen produced in an electrolytic alkali chlorate cell after removal of its substantial content of oxygen and chlorine.

2. The process of claim 1 wherein said closed electrolytic cell is provided with bipolar electrodes.

3. The process of claim 1 wherein nitrogen is added to the diluting gas.

4. The process of claim 2 wherein nitrogen is added to the diluting gas.

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS 509,023 2/52 Belgium. 488,984 1/30 Germany.

9/03 Great Britain.

OTHER REFERENCES Perry, Chemical Engineers Handbook, 3rd edition, p. 1868.

8/56 Swelheim 10 JOHN H. MACK, Primary Examiner. 3,007,854 11/6 1 Smith et a1 20439 MURRAY TILLMAN, JOSEPH REBOLD, Exa miners. 

1. A PROCESS FOR MANUFACTURING ALKALI METAL CHLORATES BY ELECTROLYSIS OF AN AQUEOUS SOLUTION OF AN ALKALI METAL CHLORIDE WHICH COMPRISES CONDUCTING THE ELECTROLYSIS IN A CLOSED ELECTROLYTIC CELL, INTRODUCING AND PASSING THROUGH THE CELL A DILUTING GAS AT SUCH A RATE THAT OXYGEN AND CHLORINE PRODUCED DURING ELECTROLYSIS ARE MAINTAINED IN A CONCENTRATION OF BELOW 6 PERCENT BY VOLUME WHILE MAINTAINING A GAS PRESSURE IN THE CELL OF AT LEAST 40 MILLIMETERS WATER PRESSURE HIGHER THAN ATMOSPHERIC PRESSURE, SAID DILUTING GAS BEING AT LEAST IN PART GASEOUS HYDROGEN PRODUCED IN AN ELECTROLYTIC ALKALI CHLORATE CELL AFTER REMOVAL OF ITS SUBSTANTIAL CONTENT OF OXYGEN AND CHLORINE. 